1. Field of the Invention
The present invention relates to a method and an apparatus for use, in the first place, within the reproduction area in the graphic industry, and more particularly for improving and simplify the possibility of preparing printing plates and print originals having correct impositions and mutual register between the sides of the front page and the rear side page for various types of printing systems.
2. Discussion of the Background
When printing any kinds of printed matters there are generally printed several pages at the same time, for instance 4, 8, 16, 32 or 64 pages on one and the same sheet, firstly with half the number of pages on one side, called the front side, and thereafter with the second half number of pages on the opposite side, called the rear side. The sheets, having been printed on both sides, are folded, are eventually joined, bound and are guillotined to form a ready printed matter.
When planning the design of the printed matter to be made the print pages have to be place according to a very accurate imposition, both so that the pages of the print sheet, after the printed matter has been folded are presented in a chronologic order, and also so that the pages on the front page and the rear page of the printed matter, that is odd and even pages, are placed exactly straight over each other. As an aid for making the layout and the succession of pages of the printed sheet correct it is necessary that the imposition is checked before the printing plates are prepared and the printing process is started. Depending on how the sheet is folded the impositions may vary. A possible example of imposition of the front and rear sides of a 16 page printing sheet is shown in FIGS. 1A and 1B.
Primarily the word “imposition” means that, for instance, 8 pages (16, 32, 64 etc) are placed in such order of a printing sheet to be printed and having a predetermined size, that the printed pages, after the sheet has been folded following in a chronologic order, so that, for instance in a 16 page sheet, the pages 1, 4, 5, 8, 9, 12, 13 and 16 are placed on the front side of the printed sheet, whereas the pages 2, 3, 6, 7, 10, 11, 14 and 15 are placed on the rear side of the sheet, and, in front of all, that the pages on the front side and rear side, respectively, are located exactly straight over each other. To that end a check print is made of the front side and the rear side of one and the same printing sheet. This is done in that a first print is made of the front page of the printing sheet, and that a second print is made of the front page of the printing sheet, and that a second print is made of the rear side of the same printing sheet, whereupon a check is made of the placing and the mutual registers of the print pages. The printing of the rear side is made after the print sheet has been turned upside down. In this connection it is of greatest importance that the print sheet is fed into the check print apparatus with an exactly correct alignment. The check printing can be made on separate sheets or from a roll of check print paper. In case of eventual misalignments between the sides a correction of the impositions must be made.
In conventional printing processes according to modem methods there are used computers for creating print pages comprising text and images. A print page is what can be seen in a book, a catalogue, a newspaper etc. Digital dates for such pages are treated in several stages resulting in a so called imposition. An imposition defines the position of each page of a printing sheet, which generally consists of 8, 16, 32 or 64 print pages. The position of each page including digital data is determined both by the printing machine, for instance an office printing machine, a gravure printing press, or in some cases a letterpress printing machine, by means of which the printing sheet is printed, and also by the way in which the print sheet is folded after having been printed. Since the printing is made on both sides of the paper there is a need for two separate impositions. A first imposition comprises digital data for printing on the front page of the print sheet, and a second imposition comprising data for printing on the rear side of the print sheet.
Methods and apparatus for printing of paper both on the front side and on the rear side are known from several different patent publications, like from U.S. Pat. No. 4,623,900, U.S. Pat. No. 4,903,043, U.S. Pat. No. 4,019,435 or from EP 367.628. In all said cases there is question of making prints or copies on both sides of a paper. There is in no case question of checking of imposition for instance for printing plates. Therefore there are no specific means for foreseeing that a paper is fed exactly correct into the printing machine in connection to printing of the front and rear sides.
In most methods, used so far, for checking the impositions it has been possible to check only one side, and this is unsatisfactory since the rear side (the completing side) is missing at the check print, and it is therefore no possibility of checking that the impositions of the front page and the rear page, respectively, of one and the same printing sheet coincide with each other.
SE518.052 having the same applicant as in the present application discloses a method and an apparatus for checking impositions whereby both the front side and the rear side are printed on one and the same printing sheet. The printing is made on separate sheets which are cut from a roll of paper. For directing the print sheet into the printer unit it is foreseen that the sheet to be printed is fed with the long sides thereof in an exactly correct position. For that purpose is used a type of buckling means, which foresees that the front edge of the print sheet, after the print sheet has been buckled, is let free so that the sheet is fed into the printing unit exactly aligned with respect to the side edges of the sheet. In the apparatus there are used two printer units, one unit for the front side of the sheet and an other printer unit for the rear side of the sheet. Aligning by means of buckling must be done at both printer units. Depending on the long transport path for the cut off sheet, which passes in the form of the numeral “8”, there appear problems with the straightening up of the sheet, in particular in connection to printing in the second printer unit, in which the “rear side” is printed.